1. Field of the Invention
This invention relates to settlers for mineral slurries (sometimes referred to as gravity settlers, pressure settlers, clarifiers, separators, thickeners, deep thickeners, and the like) used in industrial processes. More particularly, the invention relates to settlers used for decanting or thickening slurries of minerals or mineral tailings, for example red mud produced during the extraction of alumina from bauxite by the Bayer alkaline digestion process.
2. Background Art
Many industrial processes make use of tanks or reservoirs in which slurries of mineral materials or tailings are allowed to settle and densify, often with the assistance of flocculants or other chemical aids, to produce a thickened lower slurry layer and a clarified or liquid upper layer. The thickening process may be required for various reasons, but is often used to produce a thick mud or plastic solid that may be disposed of or transported more easily and economically than a thin slurry. The clarified liquid may then be re-circulated to the same industrial process or disposed of directly.
An example of a settler of this kind is disclosed in U.S. Pat. No. 4,830,507 which issued to the same assignee as the present application on May 16, 1989. The apparatus consists of a large open-topped tank having an outlet for thickened slurry at the center of the bottom wall and an outlet for clarified liquid in a side wall of the tank near the open top. Slurry to be decanted is introduced into the tank via a feed well positioned at the center of the tank near the top. The feed well is an upright cylinder consisting of a cylindrical side wall having an open upper end and a lower end that is partially closed by an annular inward projection or lip from the lower end of the side wall. The inward projection terminates short of the center of the feed well, leaving a circular opening at the center. The feed well is partially submerged beneath the upper surface of the slurry in the tank and fresh slurry is fed into the inside of the feed well below the slurry surface tangentially along the cylindrical inner wall, creating a circular flow of slurry within the feed well. A flocculant or other chemical may be added to the feed well for mixture with the slurry so that flocs may form and grow in a single location, and the flocculated slurry mixture then descends through the central opening at the bottom of the feed well into the body of the tank where settling takes place. The settling process is assisted by an upright rotating stirrer in the form of a rake or the like arranged centrally in the tank.
Settlers of this kind work well for slurries made up of relatively fine suspended particles that do not differ greatly in diameter throughout the slurry. For example, red mud from the Bayer process usually has a particle size in the range of up to 10 microns. However, it has been found that when slurries contain coarse particles, in addition to fine particles, problems can arise. For example, some slurries may contain sand or other coarse particles in addition to mud particles. Particles of sand may have diameters of more than 75 microns, and often more than 100 or even 400 microns (in fact, sand particles may even be in the 1000 to 2000 micron size range). When such slurries are decanted in conventional apparatus of the above kind, deposits of solids made up of segregated size fraction materials and more specifically of coarse particles may form in the settler tank, especially around the central lower outlet and the stirrer. Such deposits may eventually cause blockages and/or cause the stirrer to stop or may even damage the stirrer due to the application of excessive torque, and this requires a premature shut-down of the apparatus for cleaning or repair. Obviously, this causes disruption of the industrial process and delay.
There is therefore a need for improvement of settlers used for decanting slurries, particularly for slurries containing a large proportion of coarse particles.